I recently decided to install Arch on my desktop and thought I'd give KDE another go. I have been using Gnome 3 on my laptop for a while and I wanted to try something else.
One issue had been integrating the look of GTK and QT apps. I set the GTK theme to be oxygen-gtk and I set the qt widget theme to oxygen. It accepted the change without errors and the open windows would change. When I opened another window it changed back to using the GTK theme in my QT apps even though in the settings it was set to oxygen. I finally figured it out after reading this: Arch Forum
pacman -S kde-gtk-config
This allows the modification of GTK theme under Settings -> Application Appearance -> GTK. To be able to use more themes and a lovely icon set install:
pacman -S oxygen-gtk2 oxygen-gtk3 faenza-icon-theme
However it will revert back to GTK. To get it to stay permanently you need to run this command: